Fix `badarith` error in couch_db:get_db_info call

When folding we account for a previous `null`, `undefined`, or a number. However
btree:size/1 returns 0, `nil` or a number.

Switched `undefined` to `nil`. Couldn't find where btree:size would ever return
`undefined`, it seems we meant to use `nil` instead.

COUCHDB-3316
1 file changed
tree: 78f0ed0311cd76071df3d06047e4feff3abb6568
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